Well I'm still testing these mags (so far, so good) so no full review yet but here's a quick overview:

Things that are different:
-New polymer material for increased strength & durability. I can't feel a difference but Magpul says it's different
-Modified external/internal geometry so they can be used in multiple platforms to include the HK416, MR556A1, SA-80, SCAR 16/16s, M27 IAR, and possibly others
-The bolt catch cutout has been re-designed for use in the weapons above
-The slide has an overtravel stop to ensure it can't go to far into the magwell which can cause issues with the bolts on non-AR rifles
-Floorplate is slimmer to allow a better fit in dual mag pouches
-Dustcover doesn't clip on the bottom anymore; only on the top

Things that are the same:
-The proven stainless spring is the same
-The anti-tilt follower and curvature of the mag is the same. The follower is marked differently (see video) but is shaped the same
-Disassembly is the same; no tools required
-Still includes the dust/impact cover and it works on both the 'old' and new Gen M3
-Still made in the USA

Here's a video with some close ups, a bunch of additional photos at the end, and demonstrations of what I was talking about above: